Interest in metallocene and non-metallocene single-site catalysts has continued to grow rapidly in the polyolefin industry. These catalysts are more active than conventional Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, enhanced incorporation of .alpha.-olefin comonomers, and lower polymer density. Examples of non-metallocene single-site catalysts include catalysts containing a boraaryl moiety such as borabenzene, boranaphthalene or boraphenanthrene. See U.S. Pat. No. 5,554,775 and PCT Int. Appl. WO 97/23512.
Unfortunately, the uniformity of molecular weight distribution (MWD) reduces the thermal processing ability of polyolefins made with single-site catalysts. These polyolefins also have a higher tendency to melt fracture, especially at higher molecular weights. These disadvantages combine to make it difficult to process polyolefins produced by single-site catalysts under conditions normally used for Ziegler-Natta polymers. Controllable broadening of MWD is therefore a desired advance in single-site catalyst technology.
One method of increasing processability and broadening MWD of polyolefins produced by single-site catalysts is to physically mix two or more different polyolefins to produce a blended polyolefin mixture with a multimodal molecular weight distribution. For example, see U.S. Pat. No. 4,461,873. In addition, olefin polymerization has been performed in a dual reactor system in order to broaden MWD. The olefin is polymerized by a catalyst in one reactor under one set of conditions, and then the polymer is transferred to a second reactor under a different set of conditions. The first reactor typically produces a high-molecular-weight component, and the second reactor produces a low-molecular-weight component. See U.S. Pat. Nos. 4,338,424, 4,414,369, 4,420,592, and 4,703,094. Lastly, a one-reactor, two-catalyst process has also been used to make multimodal, broad-MWD polymers. The olefin is polymerized in one reactor by two catalysts with different reactivity to form a reactor blend having broad and/or multimodal molecular weight distribution. The catalysts may be either two (or more) separate metallocenes or a metallocene and a Ziegler-Natta component. See, for example, U.S. Pat. Nos. 4,937,299 and 4,530,914, in which at least two separate metallocenes are used in one reactor to form multimodal polymers. See U.S. Pat. Nos. 5,032,562 and 5,539,076 for examples of the metallocene/Zeigler-Natta catalyst mixture in one reactor.
A significant disadvantage of each of these methods is the added cost of using two reactors or two catalysts in the polymerization process. A simpler method would use a single catalyst system that produces multimodal, broad MWD, polymers in a one-reactor process. A catalyst that can produce multimodal polyolefins having separate components of distinct molecular weight would be especially valuable since these polymers are useful in blow-molding and film resin technologies.
In sum, new catalysts are needed. Particularly valuable catalysts are those that could be used in a one-reactor process to produce multimodal polyolefins having separate components of distinct molecular weight.